Finding a way to keep snoops from tapping into other people's information is a challenge that has gone to the subatomic level. First proposed in 1984, quantum cryptography (QC) promises to send coded messages that are, according to the laws of quantum mechanics, impossible to crack. The technique relies on a principle of modern physics called Heisenberg's uncertainty principle, which states that it's impossible to measure something at the subatomic level without altering it.

QC systems send information in the form of a specially prepared stream of photons representing 0s and 1s. If anyone tries to eavesdrop, he unintentionallly alters the photons being transmitted, and the rightful recipient is able to detect the tampering. As a final layer of security, the beam of photons doesn't encode the actual secret message, it just contains an encryption key. So if part of the key is intercepted, the sender and recipient detect the altered photons and discard that part of the key. Once they've transmitted enough photons, the shared key is used to encrypt the message, which can be sent over public communication lines. But the photon key has to arrive reliably at its destination.

Quantum key encryption is one promising method of securing communication, especially if it can be transmitted by satellites. Scientists at an Italian observatory this year succeeded in firing lasers at the mirror-covered Ajisai Japanese satellite, proving that a sequence of photons can travel great distances through space. The laser pulsed photons at the satellite at 17,000 times per second; a fraction bounced back to a telescope at the observatory. On Earth, the longest successful quantum encryption link has been just under 100 miles because the photons scatter as they travel through the air. To reach the satellite, the photons only had to travel through 5 miles of atmosphere during their 1000-mile journey, allowing the sequence to arrive in order.

There have been several recent breakthroughs in quantum cryptography. In August, a team of researchers from the National Institute of Standards and Technology (NIST) took the stage at Caesar's Palace in Las Vegas for a demonstration of its quantum cryptography system at the notorious Black Hat information security conference. Using a laser to send the encryption key across the room, they streamed perfectly secure live video at 300,000 bits per second--as good as YouTube. "That's about two orders of magnitude faster than any other system for quantum key distribution," says NIST engineer Alan Mink.

At the same conference, researchers from the University of Singapore demonstrated a system using pairs of "entangled" photons. Entanglement is a mind-bending feature of quantum mechanics that can allow the physical properties of two particles to be intimately linked even if they're separated by a great distance. This provides an ideal way for a third party--a satellite, for instance--to distribute a perfectly secure key to two parties who wish to exchange a message, no matter where they're located.

Last fall, a secure QC line built by Geneva-based Id Quantique was used to transmit voting data in the Swiss national elections. And New York-based MagiQ Technologies has sold "a moderate number" of systems to clients in military and intelligence agencies, financial institutions and telecom companies, says company spokesman Andrew Hammond. No clients are willing to be named publicly, he says, but the systems sell for between $125,000 and $175,000.

In keeping with the 100-mile practical limit for terrestrial QC, Hammond says MagiQ's systems are best suited for metro-area networks. The company is also in discussions about a possible Washington-to-New York link, in which the signal would be amplified at a network node partway along the route. For international distances, space may be the solution. "We're not capitalized to send up our own satellite," Hammond says, "but from an architecture standpoint, we think it makes a great deal of sense." Mink agrees: though the transmission speed via satellite would initially be very slow, the technology is evolving rapidly. "It can be done," he says. "Definitely."